The portion of the martian south polar cap that persists through each southern hemisphere summer is known as the residual cap. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows a 2.9 by 4.8 km (1.8 by 3 mi) area of the south polar residual cap as it appeared in mid-summer on 23 February 2000. The landscape of the south polar residual cap is dominated by layered, frozen carbon dioxide ("dry ice") that has been eroded into a variety of pits, troughs, buttes, and mesas. Commonly, the pits are circular and the mesa scarps are arcuate. In summer, as carbon dioxide is subliming away, the scarps bounding the pits and mesas darken. The darkened slopes may indicate that small amounts of dust are present, mixed-in with the ice. The ice is layered, indicating many cycles of deposition preceded the present period of sublimation and erosion. Recent MGS MOC images acquired in 2001 have indicated that the scarps are retreating an average of 3 meters (3.3 yards) per martian year. As more carbon dioxide is released into the atmosphere each southern spring and summer, the atmospheric pressure of Mars may increase such that it could double in a few hundred to a thousand Mars years (687 Earth days = 1 Mars year). The picture shown here is from MOC image M12-02295 and is illuminated by sunlight from the lower right.

A version of this picture appears on the cover of the December 7, 2001, issue ofScience and accompanies a paper regarding the MGS MOC discovery of evidence for martian climate change.